(1) Field of the Invention
The present invention relates to access restriction technology to prevent unauthorized access to a storage unit through a communication unit that communicates with an external communication network, which is a communication network connecting subscribers, such as telephones and data communications. For example, the present invention relates to an effective access control technology applicable to a microcomputer equipped with a communication function.
(2) Description of the Related Art
Japanese Patent No. 3591229 (Patent Literature 1) describes a system that includes a base station device and a terminal device coupled to the base station device through a communication function and performs maintenance on software of the terminal device with software downloaded from the base station device to the terminal device. This software maintenance means, in most cases, rewriting of programs or data stored in a nonvolatile storage area in the terminal device.
For the purpose of dispensing with additional memory used only to save communication subroutines transferred from a nonvolatile memory where firmware is stored in an apparatus like an electronic camera (cost reduction), and also circumventing the need to rewrite the firmware concurrently with reception of rewrite data (hang-up prevention due to a communication failure), the technology described in Japanese Unexamined Patent Application Publication No. 2004-280559 (Patent Literature 2) adopts a firmware rewrite control unit. The firmware rewrite control unit first stores firmware rewrite data transmitted from an external apparatus, such as a PC, in a first memory in the apparatus and then when the end of communication between a data rewrite device and the external apparatus is detected, rewrites the firmware stored in a second memory with the firmware rewrite data in the first memory. Internal data transfer and rewrite control sequences performed by the firmware rewrite control unit can be implemented by either programs or hardware logic.